var return_top = document.querySelector('#return_top');
return_top.onclick = function(){
    var y = window.scrollY;
    var timer = setInterval(function(){
        y-=50;
        window.scrollTo(0,y);
        if(y <= 0 ){
            clearInterval(timer);
        }
    },1)
}
window.onscroll = function(){
    if(scrollY>=300){
        return_top.style.display = 'block';
    }else{
        return_top.style.display = 'none';
    }
}
